Gordon Brown faces a new assault on his leadership this week when one of Labour's most influential figures will accuse the party of being "paralysed" by defeatism and of an "astonishing" failure to attack the Tories.

The criticism from Jon Cruddas, seen by many on Labour's centre-left as a leadership contender, is bound to re-ignite speculation over whether Brown should lead the party into the next general election. In a speech to the Compass pressure group on Tuesday, Cruddas will argue that Labour has failed completely to exploit the Tories' difficulties during a summer in which David Cameron's party has been hit by internal rows over the NHS, MPs' expenses and Europe.

The MP for Dagenham will argue that the economic downturn has exposed the Tories' "Thatcherite" instincts and presented Labour with a "golden opportunity" to expose Cameron's claim to the political centre ground as a sham.

"It is frankly astonishing, after a summer in which the Tories have showed their true colours, that we have barely laid a glove on them," Cruddas will say. "Progressive conservatism hit a wall as soon as the economic storm clouds rolled in and appears to have been dropped in favour of a harsher, nastier Thatcherism.

"But presented with this golden opportunity to galvanise ourselves and unite behind a clear anti-Tory message we seem paralysed; afraid of using our Labour principles as the basis to lay bare the shallowness of Cameron's project. We seem to be meekly accepting defeat, unable to show what we believe in."

Cruddas will insist that Labour can still win the next election "if we get our message right". But in a clear warning to Brown, he will say the party only has "months to get this right. Otherwise, we will go down to catastrophic defeat and deserve to."

With the party way behind in the polls, a growing number of Labour MPs and peers are beginning to discuss again how they might persuade Brown to step aside, possibly as late as early next year.

By that stage the party could argue that it will be too close to a spring election to hold a leadership contest and that a new leader could be ushered in by popular acclaim without a divisive fight.

Many cite the example of the Australian Labor party, which in 1983 dumped its leader, Bill Hayden, just weeks before an election, installing Bob Hawke who revived morale to such an extent that the party turned it fortunes round and won against the odds.

In a further sign of Brown's difficulties, the Observer has learned that one of his closest political allies has refused a personal appeal by the prime minister to return and oversee his election campaign, in a rebuff that will heighten Labour concerns about weaknesses in its election strategy. Spencer Livermore left Downing Street last year after more than a decade as one of Brown's most trusted lieutenants. He had been publicly – and wrongly – blamed for the prime minister's failure to hold a snap election the previous autumn.

Brown is known to have tried to lure Alastair Campbell back to run the campaign amid warnings from Peter Mandelson that his current team lacks sufficient experience. At least two other former senior Labour strategists now working in the private sector have also been sounded out about a return to Downing Street. Both declined.

"Probably the one person now who would come back is Damian McBride and I'm told that he still believes he will, though it seems impossible to imagine he could," said one former Downing Street staffer.

McBride was forced to quit Downing Street in April over his involvement in plans to spread ill-founded gossip about senior Tories on an anonymous website, and Brown has insisted that he has subsequently broken off all contact.